Can a dad with B blood have an o- child?

Yes, it is possible for a dad with B blood to have an O- child.

Here's an explanation:

Blood types are determined by the presence or absence of certain antigens on the surface of red blood cells. There are two main blood group systems: the ABO blood group system and the Rh blood group system.

In the ABO blood group system, there are four main blood types: A, B, AB, and O. Blood type O does not have any A or B antigens on the red blood cells, while blood type B has only B antigens.

The Rh blood group system involves a separate antigen, called the Rh factor. People who have the Rh factor are Rh-positive, while those who don't have it are Rh-negative.

When a child inherits blood type O from one parent, they must have received an O allele from that parent. The other allele can be either A or B, which would result in blood type A or B.

In the case of a dad with B blood and an O- child, it is possible that the father is actually heterozygous for the ABO blood group. This means he has one B allele and one O allele. When he contributes his sperm, he can contribute either the B allele or the O allele to his children.

If the father contributes the O allele, the child would inherit the O allele from both parents and have blood type O. If the father contributes the B allele, the child would inherit one B allele from the father and one O allele from the mother, resulting in blood type B.

Additionally, since the child is Rh-negative, the father must also be Rh-negative. If the father was Rh-positive, the child would be Rh-positive as well, as Rh negativity is a recessive trait.

Therefore, it is possible for a dad with B blood and Rh-negative status to have an O- child.